/Engine/Plugins/Experimental/GeometryScripting/Source/GeometryScriptingCore/Public/GeometryScript/MeshQueryFunctions.h
Compute the barycentric coordinates (A,B,C) of the Point relative to the specified TriangleID of the TargetMesh. The properties of barycentric coordinates are such that A,B,C are all positive, A+B+C=1.0, and A*Vertex1 + B*Vertex2 + C*Vertex3 = Point. So, the barycentric coordinates can be used to smoothly interpolate/blend any other per-triangle-vertex quantities. The Point must lie in the plane of the Triangle, otherwise the coordinates are somewhat meaningless (but clamped to 0-1 range to avoid catastrophic errors) The Positions of the Triangle Vertices are also returned for convenience (similar to GetTrianglePositions)
static [UDynamicMesh](API\Runtime\GeometryFramework\UDynamicMesh) * ComputeTriangleBarycentricCoords ( [UDynamicMesh](API\Runtime\GeometryFramework\UDynamicMesh) * TargetMesh, int32 TriangleID, bool & bIsValidTriangle, FVector Point, FVector & Vertex1, FVector & Vertex2, FVector & Vertex3, FVector & BarycentricCoords )